added internal naive (recursive) version of CROSSPRODUCT

HISTORY.md

@@ -11,6 +11,7 @@
 
 ### 2021-02-06
 * lists, objects, threads: new RESHAPE primitive
+* lists: added internal naive (recursive)  version of CROSSPRODUCT
 
 ### 2021-02-06
 * simplified private list.range() method

src/lists.js

@@ -122,6 +122,7 @@ var ListWatcherMorph;
     ravel()                 - answer a flat list of all atoms in all sublists
     transpose()             - answer a 2D list with rows turned into columns
     reshape()               - answer a new list formatted to the given dimensions.
+    crossproduct()          - answer a new list of all possible sublist tuples
     query()                 - answer a part of a list or multidimensionel struct
 */
 
@@ -642,6 +643,20 @@ List.prototype.asChunksOf = function (size) {
     return trg;
 };
 
+List.prototype.crossproduct = function () {
+    // expects myself to be a list of lists.
+    // answers a new list of all possible tuples
+    // with one item from each of my sublists
+    if (this.isEmpty()) {
+        return new List([new List()]);
+    }
+    return this.at(1).map(
+        first => this.cdr().crossproduct().map(
+            each => this.cons(first, each)
+        )
+    ).flatten();
+};
+
 // List conversion:
 
 List.prototype.asArray = function () {